The present invention relates generally to equipment and methods of operating equipment for semiconductor fabrication. More particularly, the invention relates to methods of improving throughput in temperature ramping.
Photoresist removal (stripping or ashing) is one of the most frequently applied processes in semiconductor industry. Due to the number of photolithographic mask steps in most fabrication process flows, it is important that ashers (strippers) attain high throughput. Removing photoresist in a vacuum chamber using an aggressive plasma chemistry is considered to be the industry standard.
Two types of system architectures for plasma ashers are widely adopted. A first architecture employs a vacuum load-lock and a wafer vacuum transfer chamber, all kept at vacuum during processing of one cassette of wafers or other workpieces. A cassette of wafers is placed in the load-lock and the load-lock is then evacuated with a vacuum pump. A wafer transfer robot then transfers the wafers from the load-lock through the transfer chamber to a process chamber where plasma is generated to remove (strip or ash) the photoresist. In the normal operating mode, the load-lock, the wafer transfer chamber and the process chamber are all constantly under vacuum. After an entire cassette of wafers is processed and transferred back to the load-lock, the cassette load-lock is then vented to atmosphere, the processed wafers are removed and a new cassette of wafers is loaded.
A second system architecture transfers wafers from the cassette in an atmosphere-to-vacuum-to-atmosphere (AVA) sequence for each individual wafer. The robot and the wafer cassette are always at atmospheric pressure. The robot transfer wafers from the wafer cassette to the process chamber. A vacuum pump then evacuates the process chamber to a certain vacuum level suitable for plasma formation. The plasma source then generates plasma to remove photoresist. After the process is complete, the process chamber is vented to atmosphere and the processed wafer is transferred back into the cassette. This architecture entails pumping down the process chamber and venting back to atmospheric pressure for each individual wafer.
The first architecture employs two additional vacuum chambers (the load lock and transfer chambers), generally requiring an additional vacuum pump to pump down two chambers, and therefore is much more expensive than the second architecture. Machines using the first architecture are usually larger and occupy more clean room floor space, which is considered to be premium commodity in a semiconductor fabrication factory. Advantageously, however, the vacuum load-lock systems of the first architecture have relatively low non-productive overhead, exhibiting high throughput.
On the other hand, the second technique (atmospheric-to-vacuum-to-atmospheric, or AVA) involves a lower initial capital expenditure and occupies less space on the clean room floor. In order to compensate for relatively higher non-productive overhead than the vacuum load-lock system, a variety of improvements have been made to the AVA system. A conventional wafer processing sequence of the AVA machine architecture is as follows:
1. The robot transfers a new wafer or other workpiece from the cassette to the process chamber and places the wafer on support pins.
2. The wafer is lowered onto a high temperature chuck (platen).
3. The chuck heats the wafer up to the desired process temperature (e.g. 250xc2x0 C.).
4. The chamber is then pumped down to a desired process or treatment pressure (e.g. 1 Torr).
5. The process gases start to flow and plasma is ignited by a plasma source.
6. After the photoresist has been removed, the chamber is then vented back to atmospheric pressure (760 Torr).
7. The robot then exchanges the processed wafer for a fresh one from the cassette while transferring the processed wafer back to the cassette.
All the above steps are highly optimized to reduce the time required to complete each step. Heat transfer between the chuck and the wafer occurs most efficiently at atmospheric pressure; therefore, wafers are usually heated up before pumping down the chamber. In one commercial system, it takes about four seconds to heat a wafer from 20xc2x0 C. to 250xc2x0 C. at atmospheric pressure. In contrast, at a low pressure (1 Torr), it takes about 60 seconds.
Rapid wafer heating at atmospheric pressure sometimes causes wafer warping, however, which can have a variety of negative effects. Wafer warping may damage the circuits that are already fabricated on the wafer. If a wafer is warped on a chuck, the wafer temperature is no longer evenly distributed. A non-uniform wafer temperature distribution results in a highly non-uniform process, since temperature is a very sensitive process variable. The wafer is usually transferred into the chamber at room temperature (20xc2x0 C.). Once inside the chamber, the wafer starts to warm up somewhat while suspended above the chuck (e.g., on lift pins). To reduce the degree of thermal shock on the wafer, and consequently prevent wafer warping, the wafer can be left suspended over the chuck for a few seconds to pre-heat the wafer before lowering the wafer onto the high temperature chuck. The wafer descent rate can also be slowed so that the wafer is warning up on its way to the chuck. Both of these options, however, also reduce throughput.
Other efforts to improve throughput focus on minimizing the time to pump the chamber down to the treatment pressure (typically 1 Torr). Conventionally, a large vacuum pump is used for a high-speed pumping. A large diameter vacuum line is used to increase the pump line conductance. Moreover, a dedicated roughing line, described in more detail in the Detailed Description of the preferred embodiments, is introduced to bypass the high resistance throttle valve and further increase the overall pump speed during pump down. A dedicated roughing line is beneficial because the throttle valve can stay at its previous throttling position, dramatically reducing the time to reach a stable treatment pressure. Without the dedicated roughing line, the throttle valve has to be wide open during pump-down to reduce the pump-down time and then rotate to its throttling position when the process gases start to flow. This can take as much as an additional five seconds to stabilize the pressure after the chamber is pumped down.
Despite incremental improvements to the speed of each sequential step in the ashing process, a need exists for further improvements to throughput for plasma asher systems.
A method is disclosed for speeding workpiece thoughput in low pressure, high temperature semiconductor processing reactor. The method includes loading a workpiece into a chamber at atmospheric pressure, bringing the chamber down to an intermediate pressure, and heating the wafer while under the intermediate pressure. The chamber is then pumped down to the operating pressure at which substrate treatment is conducted.
The preferred embodiments involve single wafer plasma ashers, where a wafer is loaded onto lift pins at a position above a wafer chuck. After placement and sealing the chamber the pressure is rapidly pumped down from a load/unload pressure (preferably atmospheric) to about 40 Torr by rapidly opening and closing an isolation valve, and the wafer is simultaneously lowered to the heated chuck. At 40 Torr, the heat transfer from the chuck to the wafer is relatively fast, but still slow enough to avoid thermal shock. In the interim, the pump line is further pumped down to operating pressure (about 1 Torr) behind the isolation valve. The chamber pressure is then again reduced by opening the isolation valve, and the wafer is processed.
It will be understood, of course, that the preferred embodiments are merely exemplary and that other intermediate pressures can be selected, in view of the disclosure herein, depending upon the system and treatment process involved.